Based on a few experiments I carried out over the weekend I'm going to add one more step to the Making A "Thing In A Bottle" project from back in March. As you can imagine, that tutorial has been getting an increasing amount of traffic as Halloween approaches.
In the vast majority of cases it isn't necessary to do anything more than what's in the existing post, but if your specimen bottles are going to be moved frequently, or sent through the mail as a gift, I would strongly recommend that you apply a layer of silicone sealant to the bottle top before adding the wax seal. First, whether you're using a cork or conventional screw-top, apply a bead of silicone along the gap between the top and the glass. Then use a cheap craft brush or toothpick to smear a thin layer all over the top. Wait for the silicone to cure and then apply the wax as normal.
This accomplishes two things. One, it significantly increases the integrity of the lid's watertight seal. Two, it provides enough flexibility between the container, lid, and the wax that changes in temperature or air pressure won't crack it. Some minor fissures are inevitable, and even desirable from an aesthetic standpoint, but any major breaks will compromise the long-term integrity of the preserved specimen and it's fluid.